I'm a bit late, but I thought I'd try my hand for the warnings program. This program will probably segfault, though it shouldn't surprise anyone -- it does a lot of pointer munging for the heck of it.
50+ warnings, mostly unique, 256 bytes of source, so about one warning every 5 characters. As a bonus, it uses no whitespace. I'm well aware this is longer than a single statement, but it is a single line with no spaces, tabs or newlines ;)
A=&A;struct{int*A};s(P){return"";{int*A;c(A);}return;}main(){int*c={1,3},q=&A,*d,r[3][3]={0};char*u;unsigned*v;l:if(d=s?d:s,v<0,*v<0<0)if(+3<<0xfffffffffU/0,q+=(void*)printf("%s%f%s",1.,s),*u<300);else;d=d++;vsscanf("",'HOLA',c);sprintf(&A,"%c%c",""[*u]);}
Using pre tags:
A=&A;struct{int*A};s(P){return"";{int*A;c(A);}return;}main(){int*c={1,3},q=&A,*d,r[3][3]={0};char*u;unsigned*v;l:if(d=s?d:s,v<0,*v<0<0)if(+3<<0xfffffffffU/0,q+=(void*)printf("%s%f%s",1.,s),*u<300);else;d=d++;vsscanf("",'HOLA',c);sprintf(&A,"%c%c",""[*u]);}> gcc -Wall warnfactory.c -Wextra -O
warnfactory.c:1: warning: type defaults to `int' in declaration of `A'
warnfactory.c:1: warning: initialization makes integer from pointer without a cast
warnfactory.c:1: warning: data definition has no type or storage class
warnfactory.c:1: warning: no semicolon at end of struct or union
warnfactory.c:1: warning: unnamed struct/union that defines no instances
warnfactory.c:1: warning: return type defaults to `int'
warnfactory.c: In function `s':
warnfactory.c:1: warning: type of "P" defaults to "int"
warnfactory.c:1: warning: return makes integer from pointer without a cast
warnfactory.c:1: warning: implicit declaration of function `c'
warnfactory.c:1: warning: `return' with no value, in function returning non-void
warnfactory.c:1: warning: this function may return with or without a value
warnfactory.c: At top level:
warnfactory.c:1: warning: unused parameter 'P'
warnfactory.c:1: warning: return type defaults to `int'
warnfactory.c: In function `main':
warnfactory.c:1: warning: initialization makes pointer from integer without a cast
warnfactory.c:1: warning: excess elements in scalar initializer
warnfactory.c:1: warning: (near initialization for `c')
warnfactory.c:1: warning: initialization makes integer from pointer without a cast
warnfactory.c:1: warning: missing braces around initializer
warnfactory.c:1: warning: (near initialization for `r[0]')
warnfactory.c:1: warning: the address of `s', will always evaluate as `true'
warnfactory.c:1: warning: pointer type mismatch in conditional expression
warnfactory.c:1: warning: ordered comparison of pointer with integer zero
warnfactory.c:1: warning: comparison of unsigned expression < 0 is always false
warnfactory.c:1: warning: comparisons like X<=Y<=Z do not have their mathematical meaning
warnfactory.c:1: warning: left-hand operand of comma expression has no effect
warnfactory.c:1: warning: integer constant is too large for "unsigned long" type
warnfactory.c:1: warning: division by zero
warnfactory.c:1: warning: implicit declaration of function `printf'
warnfactory.c:1: warning: format argument is not a pointer (arg 2)
warnfactory.c:1: warning: double format, pointer arg (arg 3)
warnfactory.c:1: warning: too few arguments for format
warnfactory.c:1: warning: assignment makes integer from pointer without a cast
warnfactory.c:1: warning: comparison is always true due to limited range of data type
warnfactory.c:1: warning: left-hand operand of comma expression has no effect
warnfactory.c:1: warning: empty body in an else-statement
warnfactory.c:1: warning: suggest explicit braces to avoid ambiguous `else'
warnfactory.c:1: warning: empty body in an if-statement
warnfactory.c:1: warning: operation on `d' may be undefined
warnfactory.c:1: warning: implicit declaration of function `vsscanf'
warnfactory.c:1:220: warning: multi-character character constant
warnfactory.c:1: warning: passing arg 2 of `vsscanf' makes pointer from integer without a cast
warnfactory.c:1: warning: passing arg 3 of `vsscanf' from incompatible pointer type
warnfactory.c:1: warning: implicit declaration of function `sprintf'
warnfactory.c:1: warning: array subscript has type `char'
warnfactory.c:1: warning: passing arg 1 of `sprintf' from incompatible pointer type
warnfactory.c:1: warning: too few arguments for format
warnfactory.c:1: warning: unused variable `r'
warnfactory.c:1: warning: label `l' defined but not used
warnfactory.c:1: warning: control reaches end of non-void function
warnfactory.c:1: warning: 'u' might be used uninitialized in this function
warnfactory.c:1:257: warning: no newline at end of file